Photographic elements are generally composed of many different materials, both photosensitive and non-photosensitive materials, coated in multiple layers. The number of layers can be considerable, especially in color imaging, with each primary color forming material present in a layer that is separated from other layers by barrier layers. In addition to the photosensitive materials such as various silver salts and leuco dye reagents, it has been found advantageous to add various other reagents that perform functions such as filtering, sensitization, development inhibition or acceleration, antihalation or antifogging, toning, image stabilization, or preservation of the imaging material during storage. It is often desirable to add a particular photographically useful reagent to a particular layer of the imaging construction, and it is usually important that the reagent not sublime or migrate into an adjoining layer prematurely. Unfortunately, it is often found that when different photographically useful reagents are added to imaging materials, they do react with each other during storage of the imaging materials prior to processing, or decompose by the action of heat or oxygen. Thus these reagents do not exhibit the enhanced performance effects that were expected. Additionally, many reagents that exhibit a desirable effect such as stabilization of an image after processing are found to have a deleterious effect on the initial sensitometric characteristics of the imaging material.
As a means of addressing these problems, reactive photographically useful reagents have been blocked with various modifying groups. These blocking or modifying groups convert the reactive photographically useful reagent into a form that is more stable, or inert, or less migratable or diffusible. However, these blocking or modifying groups are generally designed such that they are removed from the photographically useful reagent at some stage of processing which may, for example, involve heating or treatment with alkali. Thus, the blocked or protected photographically useful reagent is actually a precursor reagent from which the photographically useful reagent is liberated at some point during processing.
Various blocking techniques have been utilized. U.S. Pat. No. 3,615,617 describes acyl blocked photographically useful reagents. U.S. Pat. Nos. 3,674,478 and 3,993,661 describe hydroxyarylmethyl blocking groups. Thiocarbonate blocking groups are described in U.S. Pat. No. 3,791,830, and thioether blocking groups in U.S. Pat. Nos. 4,335,200, 4,416,977 and 4,420,554. useful reagents which are blocked as urea or thiourea derivatives are described in U.S. Pat. No. 4,310,612. Blocked imidomethyl derivatives are described in U.S. Pat. No. 4,350,752, and imide or thioimide derivatives are described in U.S. Pat. No. 4,888,268. Removal of all of these aforementioned blocking groups from the photographically useful reagents is accomplished by an increase of pH during alkaline processing conditions of the exposed imaging material.
Other blocking groups which are thermally sensitive have also been utilized. These blocking groups are removed by heating the imaging material during processing. Photographically useful reagents blocked as thermally-sensitive carbamate derivatives are described in U.S. Pat. Nos. 3,844,797 and 4,144,072. These carbamate derivatives presumably regenerate the photographic reagent through loss of an isocyanate. Hydroxymethyl blocked photographic reagents which are unblocked through loss of formaldehyde during heating are described in U.S. Pat. No. 4,510,236. Photographic reagents which are blocked by a Michael-type addition to the carbon-carbon double bond of either acrylonitrile or alkyl acrylates are described in U.S. Pat. Nos. 4,009,029 and 4,511,644, respectively. Heating of these blocked derivatives causes unblocking by a retro-Michael reaction.
Various disadvantages attend these different blocking techniques. Highly basic solutions which are necessary to cause deblocking of the alkali sensitive blocked derivatives are corrosive and irritating to the skin. With the photographic reagents which are blocked with a heat removable group, it is often found that the liberated reagent or by-product, for example, acrylonitrile, can react with other components of the imaging construction and cause adverse effects.